1. Field of the Invention
The invention relates generally to spectrometers, and more particularly to an apparatus for providing real-time radiometric calibration of narrowband radiometry utilized for real time detection, recording, and display of mineralogic spectra in the form of radiated and reflected emissions for airborne geological investigations.
2. Description of the Prior Art
An airborne system for geographical surveys of the earth's surface using a high spectral resolution remote sensing instrument is described by W. E. Collins and S -H. Chang in "The Geophysical Environmental Research Corp. 63 Channel Airborne Imaging Spectrometer And 12 Band Thermal Scanner", SPIE Vol. 1298, pp. 62-71, Apr. 16-17, 1990. The GER imaging spectrometer covers 0.4-2.5 um in 63 channels of varying bandwidth. Both visible light and infrared detection is provided. Scanning optics utilize a rotating polygon mirror which scans the ground four times per revolution, allowing a factor of four reduction of rotation speed to scan speed. The spectrometer design provides for permanent registration between channels without adjustment. All 63 detectors have parallel preamplifiers and analog-to-digital converters. No switching or multiplexing of the detector outputs is used. A 12 bit or greater analog to digital converter permits a system providing high dynamic range with no on-board gain adjustment required. Multiple data channels may be displayed in real time during flight and recorded for later data reduction.
The present invention discloses an improved model of the GER 63 channel scanner with the capability for up to 640 channels over a range of 0.35 um through 20 um. In this embodiment a technique referred to as line or one-dimensional spectroscopy is utilized, in which radiated or reflected energy ranging from ultraviolet to infrared emanation from a terrain under observation is collimated and dispersed by diffraction gratings to provide a line image spectral display. There is a direct correlation between the detected line images and the physical configuration of the radiation source. By scanning over the surface of the terrain in a moving vehicle, preferably an aircraft, a spectral map can be obtained.
A rotating mirror is disposed to form a line image transverse to the flight path of an aircraft carrying the apparatus. For maximum spectral resolution, a plurality of spectral bands with corresponding line array detectors are provided. However, as the bandpass of a given spectral band is narrowed to provide enhanced resolution, there results an increase in attenuation of the perceived spectral energy. Further, the scanning mirror intercepts only a fraction of the available energy at any instant, thereby limiting the threshold sensitivity of the apparatus.
A further improvement provides means for generating a line image with the mirror fixed at a predetermined angle with respect to the observed scene, in what is referred to as a "staring" or "pushbroom" mode, in which the apparatus generates a field of information without relative motion between the scan mirror and the detection array, relying on the translation of the mirror and a two-dimensional array of imaging elements with respect to the terrain. Further, by scanning the terrain at a plurality of fixed mirror angular positions, data with respect to polarization of the spectral emissions may also be obtained.
In a further improvement, the scene is scanned by the rotating mirror and the image sequentially applied to the two-dimensional array so as to generate a continuous three-dimensional spectral display, in which the spatial data and spectral distribution with respect to wavelength are correlated over a substantially contiguous spectrum, while providing enhanced sensitivity and increased dynamic range over a wide field of view.
Prior art spectrographic imagers utilizing two-dimensional detection arrays are incapable of providing a continuous spectral distribution along with a continuous image display, allowing only a continuous image with selective spectral bands, or a continuous spectrum over a limited image capture swath.
Further, prior art spectrographic imagers required pre-flight and post-flight calibration with standard light sources. The prior art has attempted real-time calibration by scanning a white plate reflective target and a plurality of black body targets at the extremes of the field of view. However, disadvantageously, responsivity is frequently nonuniform over the spectral band due to the combined effects of the spectral characteristics of the mirrors, lenses, prisms, gratings and detectors. Further, such a procedure was inoperative in the staring mode. The present invention provides an on-board real-time calibration capability, utilizing a white body reflective plate or black body absorber and a radiometrically and spectrally calibrated light-emitting source providing at least one predetermined wavelength of known radiance, and wherein calibration data is recorded during each line scan of the detector array in the scan mode, and during each pixel scanned in the staring mode.